According to the Shulchan Aruch, first-born women must also fast on Erev Pesach and this is possibly the custom amongst some sefaradi women. Also if the father is a first-born and so is his infant son, then the mother should fast for her son. There are discussions regarding whether such women participate in such a siyum. In Pressburg (Bratislava) the women would participate. (Piskei Teshuvos vol.5 chap.570 fn.21) Rav Ovadiah Yosef says it is good for such women to hear the siyum from the ezras nashim, but it is also sufficient to bring some cake home from the siyum for them. (Yalkut Yosef  - Kitzur Shulchan Aruch 570:2)